code: def test_empty_col_sort(df):
      """
      Raise ValueError if a string is provided,
      but the relevant column is all nulls.
      """
      with pytest.raises(ValueError):
          df["col1"] = np.nan
          df.encode_categorical(col1="sort")
  
imports: import datetime
  import numpy
  import pandas
  import pytest
  from hypothesis import given, settings
  from pandas.testing import assert_frame_equal
  from janitor.testing_utils.strategies import categoricaldf_strategy, df_strategy
label_desc: Use image node_id 18 for calling a global function with example usage: test_empty_col_sort(df) without return types